Queuosine modification in tRNA and expression of the nitrate reductase in Escherichia coli.

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In eubacteria the modified nucleoside queuosine is present in tRNAAsn, tRNAAsp, tRNAHis and tRNATyr. A precursor of queuine, pre-queuine, is synthesized from GTP, inserted into the first position of the anticodon of the corresponding tRNAs by a specific tRNA-guanine transglycosylase and further modified to queuosine. Isogenic pairs of Escherichia coli, containing or lacking the tRNA-transglycosylase (JE 7335, tgt+ lacZ+ and JE 7337, tgt- lacZ+; JE 7334, tgt+ lacZ- and JE 7336, tgt- lacZ-), have been employed to study the function of queuosine in tRNA. Compared with the tgt+ strain (JE 7335), the tgt- mutant (JE 7337) grown under anaerobic conditions, is defective with respect to the nitrate respiration system, in which electrons are transported from D(-)-lactate via quinone and cytochrome bNO3-(556) to nitrate. Low temperature cytochrome spectra of the anaerobically grown tgt- mutant show a lowered amount of type b cytochromes involving the spectrum of cytochrome bNO3-(556). In the case of the anaerobically grown tgt- mutant three proteins are missing in the protein pattern of cytoplasmic membranes. Their mol. wts. correspond to those of the subunits of the nitrate reductase complex. In contrast to the tgt+ strains (JE 7334, JE 7335) both tgt- mutants (JE 7336, JE 7337) cannot grow on lactate under anaerobic conditions with nitrate offered as electron acceptor and NO3- is not reduced to NO2-. A possible link between Q-modification of tRNAs, the synthesis of proteins of the nitrate reductase complex and the synthesis of menaquinone or ubiquinone is discussed.

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